It is well known that manganese dioxide of a grade suitable for use as a depolarizer in dry cell batteries can be manufactured synthetically from manganese sulfate solutions by the electrolytic deposition of the dioxide onto a suitable anode. Also, it is known that the precursor manganese sulfate solutions can be prepared by the reduction or calcining and digestion or leaching of naturally occurring manganese ores such as those of the pyrolusite/cryptomelane type. However, a drawback to the use of such manganese ores of the cryptomelane type is the presence therein of high levels of potassium.
The presence of potassium impurity in electrolytic manganese dioxide adversely affects the latter's capability to function satisfactorily as a depolarizer in dry cell batteries. As a result, numerous solutions have been proposed for removing potassium impurity from electrolytic manganese dioxide, the majority of these relating to the removal of potassium impurity during the preparation of the precursor manganese sulfate solution. For example, in U.S. Pat. No. 3,667,906 there is disclosed a method of removing potassium impurity from naturally occurring manganese ores used in the preparation of the manganese sulfate electrolyte comprising washing a reduced or calcined manganese ore with hot water to remove soluble potassium prior to subjecting said reduced or calcined manganese ore to subsequent acid leaching and separation steps.
U.S. Pat. No. 4,285,913 offers yet another method for producing manganese sulfate electrolyte solutions containing reduced levels of potassium. In this patent, there is described a process wherein reduced or calcined manganese ore containing potassium impurity is leached with a liquor comprising spent electrolyte (i.e., an aqueous solution containing sulfuric acid and manganese ion) to which has been added a source of iron to provide soluble ferric ions. The leaching process is carried out under such specific conditions of pH, time and temperature as to yield a mixture of digested ore and manganese sulfate solution which solution is stated to have reduced levels of potassium impurity. Subsequent treatment of this mixture with additional reduced or calcined ore to raise the pH of the mixture and to precipitate the remaining iron followed by a liquid/solid separation step thus provides an electrolyte suited to the preparation of electrolytic manganese dioxide.
Although not mentioned by U.S. Pat. No. 4,285,913, the invention therein described apparently makes use of the "jarosite process", named after the resulting iron precipitate, as described in U.S. Pat. Nos. 3,434,947 and 3,493,365 and employed in the zinc industry for the recovery of zinc and other valuable metals by electrolytic processes. In this process, small amounts of ammonium, sodium or potassium ion are added to a leach liquor resulting from the leaching of a zinc calcine, neutralized and the iron impurity present in the solution precipitated out as jarosite having the formula EQU (NH.sub.4, Na or K)FE.sub.3 (SO.sub.4).sub.2 (OH).sub.6
according to the following reaction: EQU 3Fe.sub.2 (SO.sub.4).sub.3 +2(NH.sub.4, Na or K)OH+10H.sub.2 O.fwdarw. EQU 2(NH.sub.4, Na or K)Fe.sub.3 (SO.sub.4).sub.2 (OH).sub.6 +5H.sub.2 SO.sub.4
Of course, in U.S. Pat. No. 4,285,913, the objective is to remove potassium impurity, and this is apparently accomplished by the addition of a source of iron to the leach liquor.
While the processes of the above-noted patents may be effective in substantially reducing the levels of potassium impurity they still leave much to be desired from the standpoint of the rate at and extent to which potassium removal can be achieved. The present invention provides improvements over the processes noted above and particularly the process described in U.S. Pat. No. 4,285,913.